1. Field of the Invention
The present invention relates generally to a recording apparatus which records characters, images or the like on a record medium, and more particularly to a recording apparatus which has a plurality of recording elements.
2. Description of the Prior Art
As a recording apparatus of the above type, a thermal-sensitive printer is known which uses as a recording head a thermal head composed of a multiplicity of heater elements arranged in a line.
FIG. 1 illustrates one example of the structure of the thermal head with a driver therefor. In the figure, reference characters R1-R1,728 denote 1,728 such heater elements arranged in a line, thereby constituting a thermal head. One end of each of the heater elements R1-R1,728 is connected to a common terminal C and the other end of each heater element is connected to the output of a corresponding one of AND gates A1-A1,728 which are grouped into 4 blocks in the order in which the gates are arranged. One input to each of the blocks is connected in parallel with signal lines B1-B4 while the other input is connected in parallel with a shift register and latch circuit 100.
The shift register and latch circuit 100 includes a 1,728-bit shift register which sequentially shifts and stores recorded data transferred in series via a signal line S1, and a latch circuit which latches the data stored in the shift register. It has signal lines S1-S3 connected to its input.
Recording will be performed by use of the above structure as follows: First, a record data signal DIN is input in series via the signal line S1 to the shift register latch circuit 100. A shift clock signal SCLK to shift the data is input via the signal line S2. The input data are shifted sequentially from left to right through the shift register by the shift clock signal SCLK.
In this way, when transfer of the 1,728-bit data is completed, a latch signal LA input via the signal line S3 latches the data in the shift register into the latch circuit. The respective latched 1,728-bit data are delivered to the corresponding AND gates A-A1,728.
Strobe signals STB1-STB4 which are a block drive signal are delivered sequentially via the signal lines B1-B4 to the respective blocks of the AND gates A1-A1,728, thereby opening and closing the respective blocks of the AND gates A1-A1,728 sequentially. This causes the record data signal DIN to be delivered sequentially to the respective blocks of the AND gates A1-A1,728. This in turn causes the respective heater elements to generate heat in accordance with the corresponding data signals to record all the data in one line.
The drive circuit which drives the thermal head has a structure such as shown in FIG. 2.
In the same figure, reference numeral 200 denotes on output IC which is connected with a central processing unit (CPU) (not shown) which controls the whole recording apparatus. The output IC is controlled by a control signal CONT input via a signal line 201 from the CPU. Also, it is supplied with a record data signal having a predetermined number of parallel bits, DATA.
The output IC has first-third output ports 203-205. The first output port 203 is connected to a P/S converter 206 which converts the parallel recorded data signal DATA into the series recorded data signal DIN and outputs the signal DIN to the thermal head. The second output port 204 is connected with a shift clock controller 207 which outputs the above-mentioned shift clock signal SCLK to the thermal head and outputs a clock signal CLK which is the same as the clock signal SCLK to the P/S converter 206. The second output port 204 also outputs the latch signal LA to the thermal head. The third output port 205 is connected to a strobe amplifier 208 which amplifies the above-mentioned strobe signals STB1-4 output from the port 205 and which outputs the amplified output to the thermal head.
The drive circuit which has the above structure operates as follows.
First, the output IC 200 takes parallel record data signal DATA as it stands, input via the data bus 202, into the output port 203 in accordance with control signal CONT.
The second output port 204 outputs a control signal to instruct a P/S conversion to the P/S converter 206, thereby causing same to take in the parallel record data signal DATA from the first port 203.
Next, the second output port 204 delivers a control signal to the shift clock controller 207 to actuate same. This causes clock signal CLK to be output to the P/S converter 206 which then performs the P/S conversion of the signal DATA in accordance with the clock signal CLK, thereby outputting the series record data DIN from the P/S converter 206 to the thermal head. At the same time, shift clock signal SCLK is outputted to the thermal head, thereby causing the record data to be taken in and shifted at the shift register. When transfer of the 1,728-bit record data to the shift register is completed by repetition of the 1,728-bit record data, the output IC 200 outputs latch signal LA from the second output port 204 to the thermal head, thereby causing the latch circuit to latch the data.
The output IC 200 then outputs strobe signals STB1-4 from the third output port 205 to the strobe amplifier 208 which then amplifiers the strobe signals and supplies them to the thermal head in order to record the data, as mentioned above. The strobe signals STB1-4 take the form of a pulse, the width of which corresponds to the time for which each of the heater elements R1-R1,728 of the thermal head is supplied with electric power. The width of the pulse is also set to such a value that an appropriate record density is obtained as that each heater element is not overheated.
The arrangement is such that the whole drive operation is controlled by the software for the CPU.
According to such a structure, however, for example, temporary interruption of the input voltages, external noise imposed on the input voltages or a lightning surge, may cause the CPU to lose control. As a result, the strobe signals STB1-4 may be of d.c. output, so that the time for which the heater elements are supplied with electric power becomes longer. This degrades the quality of the recorded image, and in an extreme case, heater elements are damaged.
The above drawbacks are not only in the above thermal-sensitive printers, but also in other recording devices, such as ink jet printers, in which a plurality of recording elements each are driven by the logical product of the record data signal and the drive signal.